Mangroves reduce coastal damage from tropical storms

Ede, the Netherlands - Mangroves can help protect coastal communities by reducing the height and power of waves generated by storms, and by reducing coastal flooding during tropical cyclones, a new report by The Nature Conservancy and Wetlands International reveals. Added to other roles in erosion protection and diminishing the power of waves, mangroves can therefore play an important role in coastal defence and disaster risk reduction.

Mangroves can help protect human lives and property by reducing the impacts of storm surges, the new report exposes. Storm surges – raised sea water levels driven by winds and low pressures – are among the most devastating natural hazards in modern times and their impacts are being increasingly felt, with growing populations and infrastructure in low-lying coastal areas. Climate change may further increase the frequency or intensity of storm surges in some areas.

Recent cyclones across the Caribbean and the Philippines have served to heighten the urgency for coastal protection and disaster risk reduction. “Most countries are aware of the threat, but the default response is often to look for engineered solutions” said Dr Mark Spalding, senior marine scientist at The Nature Conservancy. “These can be highly effective, but the role of natural solutions should not be overlooked. Mangroves can be effective in reducing waves and storm surges, and can be used alongside other risk reduction measures such as early warning systems and engineered coastal defence structures”.

Reduce water levels

Mangroves can reduce water levels by up to half a metre for each kilometre of mangrove that the storm surge passes through. This implies that relatively wide mangrove belts will be needed to reduce storm surges. Such wide mangroves belts are still present in many parts of the tropics that are affected by cyclones and storm surges, such as the U.S.A., Bangladesh, India, Indonesia and Australia.

“This report shows that conservation and restoration of mangroves in vulnerable coastal areas can contribute to a risk reduction strategy to protect people and their property against inundation and damage,” said Nyoman Suryadiputra, Director of Wetlands International Indonesia. “Moreover, we observe that mangroves help people recover after coastal disasters by providing firewood, building materials and food sources, such as fish and shellfish.”

Reduce flooding from storm surges

There is little information on storm surge reduction by mangroves, because it is difficult to measure water levels during cyclones. However, mathematical models suggest that mangroves can significantly reduce flooding from storm surges in some areas. They also indicate that the most dramatic reduction in surge height may be in the front areas of the mangrove – those reached first by the surge itself. Dense mangrove forests, including species with aerial roots, are likely to be more effective at reducing water levels in a surge.

“Further research is needed to allow us to understand and quantify the coastal defence services provided by mangrove forests against storm surges. Such information is vital if we are to ensure that mangroves are used appropriately in coastal defence”, said lead author of the report Dr Anna McIvor of The Nature Conservancy.

Notes to editors:

1.The Nature Conservancy is a leading conservation organisation working around the world to protect ecologically important lands and waters for nature and people.

The Conservancy and its members have protected more than 480,000 sq km of land and is engaged in more than100 marine conservation projects in 24 countries, including the Caribbean and the Coral Triangle. The Conservancy is also developing policy and decision support tools for ecosystem-based adaptation and hazard reduction in the coastal zone (www.nature.org).

2.Wetlands International is the only global not-for-profit organisation dedicated to the conservation and restoration of wetlands. We are deeply concerned about the loss and deterioration of wetlands such as lakes, marshes and rivers. Our vision is a world where wetlands are treasured and nurtured for their beauty, the life they support and the resources they provide (www.wetlands.org).